Photo detectors are employed in many applications requiring low intensity radiation detection, such as medical imaging systems (e.g., positron emission tomography (PET)), homeland security systems, experimental high energy physics and other systems and technology. Important considerations in the selection of photo detectors are intrinsic gain, linearity and dynamic range, accuracy, recovery time, and background conditions and also signal processing, especially for the large scale detecting systems. High intrinsic gain in low intensity radiation detection allows for high performance detection, e.g., up to single photon sensitivity at room temperature and high signal-to-noise ratios, and reduces the requirements to front end electronics. Linearity and high dynamic range are important towards providing accurate measurements of the radiation energy absorbed, for example, by scintillation materials in medical imaging systems. High accuracy measurements permit discrimination of gamma-rays that have been scattered within a material or a (patient's) body. Scattered gamma-rays provide less reliable information about the distribution of gamma-ray sources in the patient's body than unscattered gamma-rays. Short recovery times are important towards achieving good performance in high gamma-ray environments, such as in cardiac scans with short-lived radioisotopes (e.g., Rb-82). Short response times are important in order to accurately measure the gamma-ray detection time, which is of particular interest in PET scanner operations, where locations of gamma ray emissions are determined by the coincident detection of gamma rays by a pair of detectors. Background conditions can contribute to “dark current events,” such as generated by the thermally created carriers inside a sensitive area. A dark current event is typically attributable to defects in or thermal variations experienced by a photo detector. Dark current events often cannot be distinguished from the intended signal generated by the detection of a photon in micro cell. Dark rate is best kept at a minimum in order to yield high signal-to-noise ratio.
Other important considerations for many low intensity radiation detection applications include operation conditions, stability, expected working conditions (e.g., a high electromagnetic and radiation field environment), physical size and fabrication cost. Operation conditions such as low bias voltage stability are important, especially for interventional devices (e.g., intra-operative cameras), in order to reduce the possibility of electric shock to a patient, sensitive technology (e.g., space technology), and mobile systems. The photo detector fabrication cost represents a significant fraction of the expense associated with the medical imaging systems, and therefore is desirably minimized.
Low intensity photon flux detection generally employs conventional photomultiplier tubes (PMT) and the related hybrid photon detector (HPD) technology. These technologies have several advantages, such as high gain (e.g., one million or higher), good linearity, and low dark current. However, these conventional technologies also have disadvantages, such as large size, high voltage, sensitivity to ambient magnetic fields, complexity, an analog operation mode and analog output signals, and high expense.
The publication of V. Golovin and V. Saveliev, entitled “Novel type of avalanche photodetector with Geiger mode operation,” Nuclear Instruments and Methods in Physics Research” 518 (2004) 560-564 discloses an semiconductor avalanche photodetector structure having multiple Geiger mode operation cells with implemented quenching mechanisms and a common electrode, referred to as a silicon photomultiplier. The silicon photomultiplier is described as a plurality of avalanche diodes (also referred to as micro-cells) on a single substrate with an implemented quenching mechanism (resistive layer) and a common electrode. An absorbed photon entering the micro-cell generates an electron-hole pair. Due to a high electric field inside of the micro-cell, a drifting electron can generate a large number of electron-hole pairs via an avalanche process, resulting in breakdown process of the pn junction of micro-cell. The resistive layer is covered over the avalanche structure of the micro-cell for the purpose of quenching the avalanche process in the micro-cell. The common electrode provides the proportional mode of detecting of low photon flux. The performance of this semiconductor structures is comparable with or higher than the performance of photomultiplier tubes, permitting these semiconductor devices to be substituted for the photomultipliers tubes of old vacuum glass radio device technology.
A new approach presented herein is based on the methods and structures which use the binary (digital) feature of the quantum photodetector structures for the signal processing in the sensor structure and provides a wide range of improvement with respect to the structure itself and signal processing.